The invention relates to a method for producing, in particular tempering, a low-emissivity layer system, and a device for carrying out the method.
The invention relates to the production, in particular to the tempering, of low-emissivity, thin layers, e.g. silver layers, which are used in the field of thermal insulation of window and façade glasses. The specific low-emissivity coatings, also called low-e coatings for short, are used for reducing heat transfer. The low-e coating is distinguished by the fact that it has a low thermal emissivity and the coating is moreover largely transparent in the visible spectral range. With the thermally insulating coatings the aim is to ensure, on the one hand, that the solar radiation can pass through the pane and heat the building, while only little heat at room temperature is emitted from the building to the environment. In a further application, the low-e coating is intended to prevent an energy input from the outside toward the inside.
The coatings used for this purpose comprise for example transparent, metallic systems, in particular silver-based multilayer systems, which have a low emissivity and thus a high reflection in the infrared range of light, in conjunction with a high transmissivity of the entire layer system in the visible spectral range. The metallic layers usually used are used with such a thickness with which they still have the required transparency. For example, silver up to a thickness of 20 nm is still regarded as transparent. The transparent metallic layers having high reflection in the IR range are generally designated as IR reflection layers, for differentiation.
By contrast, glass and other nonmetallic substrate materials generally have a high emissivity in the infrared spectral range. This means that they absorb a high proportion of the thermal radiation from the environment and at the same time, according to their temperature, also emit a large amount of heat to the environment.
The method used for producing a low-e coating of the substrate is generally a vacuum method, such as evaporation methods or sputtering technology. However, these thin layers usually cannot be deposited ideally conformally and tend toward dewetting, which results in a corrugated, i.e. nonuniform, layer thickness distribution. However, this energetic limitation of growth can be partly compensated for by the top layers, with the result that diffusion processes and the leveling of the silver layers occur during a downstream temperature increase. This results from the shift in the surface energy equilibrium in favor of a wetted configuration. These layers having a homogeneous thickness are distinguished by a corresponding decrease in the sheet resistance and afford the advantage of increased reflection in the infrared range of light and thus a reduced emissivity.
This effect is known from the production of safety glass. The customary procedure for this purpose, in a so-called tempering process, is for the already coated glasses to be greatly heated above their softening point, typically to 680-720° C., and then rapidly cooled and thus thermally prestressed. However, since this means additional costs, the panes processed to form safety glass are generally only those for which this is prescribed for their use. A large proportion of the panes remain untreated in this regard. In the course of this heat treatment process, however, the optical properties of the multilayer system, such as e.g. the reflection color or transmission, in particular in the visible range of the electromagnetic spectrum, also change as a result of temperature-dictated diffusion processes and chemical reactions. These changes are disadvantageous, however, since untreated and treated panes are installed alongside one another for cost reasons, with optical differences being extremely disturbing. According to the prior art, therefore, attempts are made to fashion the low-e layer systems in such a way that the changes in the optical and thermal layer properties on account of the heat treatment of the coated substrate remain minimal, at least in a range such that visually no difference can be ascertained.
Subsequently, these thermally prestressed substrates are no longer configurable. That means that, in contrast to what is customary for glass, they can no longer be shaped by means of scribing and breaking or mechanically processed in some other way. Furthermore, microscopic defects such as microcracks in panes treated in this way can lead to spontaneous cracking. In order to prevent this risk, said panes for specific applications have to be subjected to a heat soak test, i.e. a test involving the heat soak process for single-pane safety glass.
In order to ensure the configurability of the glass, endeavors are made, in RTP, to heat only the functional layer, i.e. the low-e layer, alone, without changing the substrate. The term “RTP” (“rapid thermal processing”) is taken to mean a rapid thermal treatment. The prior art discloses in this respect experiments with lasers, for example from the document WO 2010/142926 A1, which operate in the near infrared range, called IR range below. In addition to the relatively far IR range, the low-e layers absorb sufficiently well in the UV range, too. For both absorption ranges of the low-e layer system, however, the use of linear lasers comprising semiconductor diodes for brief tempering is very cost-intensive, which is disadvantageous.